抄録
Based on the geological research of the Enrei Formation in the Enrei-Tokawa area in the southern part of Median Uplift Belt of the North Fossa Magna region, central Japan, we discuss here the late Early to Middle Pleistocene tectonics in and around this area including the motion on the central part of the left-lateral Itoigawa-Shizuoka Tectonic Line active fault system (ISTL-AFS). The ISTL-AFS runs through the southwestern margin of this area and is situated in the connecting portion between the NNW-SSE ~ N-S trending northern segment in the Matsumoto Basin and the NW-SE trending southeastern segment in the Suwa Basin. The Enrei Formation here has steeply to moderately dipping chaotic structures without distinctive preferred orientations in map-scale, in contrast with the overall near-horizontal structures of this Formation and the coeval Komoro Group outside of the study area. The complicated horizontal shortening to form some folds with gently-plunging axes and irregular block-rotations about vertical axes probably formed this structural architecture. The age and space relationships between the Enrei Formation and the ISTL-AFS suggest that the chaotic structures were formed in association with the left-lateral strike-slip motion on the ISTL-AFS that formed the pull-apart Suwa Basin after 0.8 Ma and before 0.4 Ma. During and after this event, this area uplifted and eroded to form a gentle relief surface. The surface on the western side was then covered by the younger gravels derived from the Chikuma Mountains on the east that have uplifted at the average rate of about 2.5 m/ky. since 0.4 Ma. The activity of N-S ~ NNE-SSW trending high-angled faults, the Gakenoyu-Midoriko Faults, played an important role in this uplift. These sequential events including horizontal shortening, vertical-axis rotations, faultings, and rapid uplifting of the Chikuma Mountains were probably caused under the transpressional regime assosicated with the restraining bend of the left-lateral ISTL-AFS. Marked differences in tectonic landforms on both sides of the Suwa Basin are also responsible for its lateral motion.
